The optimal control and allocation of the elevators in an elevator group is contingent on, inter alia, the group control of the elevator group having sufficiently accurate data for calculating and predicting the run times of the elevators. The parameters to be used in calculating run times are generally determined manually and recorded e.g. in the group control permanently. It is also possible that the group control receives some of the data needed in the calculation of the run times from other control apparatuses of the elevator group, e.g. from elevator-specific control apparatuses (from the elevator controls). When modernizing elevators, the parameters connected to run times are not generally known and often it is necessary to give rough default values for them. This easily results in under-utilization of the transport capacity of the elevator group, especially in the starting phase of modernization, because the new modernized group control is not able to distribute calls optimally between the modernized and the unmodernized elevators. In addition, it is necessary to make complex connections between the old group control and the new group control and also to install a possibly large plurality of sensors detecting the position at cetera, of the elevator cars so that the new group control would have sufficient position data and status data about the elevators to be modernized for controlling the elevators during the modernization.